1. Field of the Invention
The present invention relates to an optical receiver for receiving optical signals with high speed modulation in an optical communication system.
2. Description of the Background Art
A conventionally known optical receiver adopts a direct detection scheme to convert the optical signals directly into the electric signals by means of a photo-detector element such as a photo-diode. Consequently, in a conventional optical receiver, the bandwidth of the receivable optical signals is limited to the bandwidth of the photo-detector element.
In this regard, although the homodyne detection scheme used in a coherent optical communication can improve the reception sensitivity in comparison with the direct detection scheme mentioned above, in this homodyne detection scheme, the bandwidth of the receivable optical signals is similarly limited to that of the bandwidth of the photo-detector element or the bandwidth of the other element such as a discrimination circuit for processing the electric signals obtained by the photo-detector element.
Consequently, the conventional optical receiver has a limited response speed, and it has conventionally been quite difficult both technically as well as economically to realize an optical receiver capable of receiving the optical signals with high speed modulation in forms of the so called optical cells or optical packets for carrying a finite bit-sequence within a finite temporal length T.
More specifically, one example of a conventional ultrashort light pulse measurement device is disclosed in Y. Ishida et al., "Self-Phase Modulation in Hybridly Mode-Locked CW Dye Lasers", IEEE Journal of Quantum Electronics, Vol. QE-21, No. 1, Jan. 1985, pp. 69-17, which has a schematic configuration as shown in FIG. 1.
In this conventional device of FIG. 1, the ultrashort light pulse is split by a half mirror 101 into two parts, and one of which is reflected by a fixed mirror 102 while another one of which is reflected by a movable mirror 103 to give a variable delay. These two parts of the ultrashort light pulse are then focused by a lens 104 to a KDP crystal for generating a second harmonic. At this point, due to the variable delay given at the movable mirror 103, there is a time difference between the arrival times of the two parts. The second harmonic generated at the KDP crystal is then entered into a monochromator 108 through a filter 106 and a lens 107, so as to observe the resulting spectrum while varying the variable delay. The original ultrashort light pulse can then be reconstructed from the observed spectrum of the second harmonic.
However, this conventional device of FIG. 1 is associated with the problem that it is only applicable to the light pulses that can be generated stably for a number of times repeatedly, because it is necessary to observe the spectrum of the second harmonic while varying the variable delay.
In addition, this conventional device of FIG. 1 is also associated with the problem that it requires a relatively large light pulse optical power (normally over 0.1 to 1 W) because of its use of the second harmonic generation which is the nonlinear optical phenomenon.
Because of these problems, it is quite difficult to utilize this conventional device of FIG. 1 for a highly sensitive reception of a single optical signal, and in particular, it is highly implausible to consider its application to the optical communication system.
On the other hand, another example of a conventional ultrashort light pulse measurement device is disclosed in Y. Ishida et al., "A Simple Monitoring System for Single Subpicosecond Laser Pulses using an SH Spatial Autocorrelation Method and a CCD Image Sensor", Optics Communications, Vol. 56, No. 1, November 1985, pp. 60, which has a schematic configuration as shown in FIG. 2.
In this conventional device of FIG. 2, the ultrashort light pulse is split into two parts, and the spectrum of the second harmonic is observed as a function of pulse arrival time difference, Just as in the conventional device of FIG. 1 described above. Namely, the ultrashort light pulse is split into two parts by a half mirror 111 and lead to a KDP crystal 113 by a mirror 112-1 and 112-2 on one hand and by a mirror 112-3 on the other hand via different routes having different path lengths, so as to give the time difference between the arrival times for two parts. Then, the second harmonic generated at the KDP crystal 113 is entered into a monochromator 115 through a filter 114, so as to observe the resulting spectrum. The original ultrashort light pulse can then be reconstructed from the observed spectrum of the second harmonic.
This conventional device of FIG. 2 has an advantage over the conventional device of FIG. 1 in that the pulse arrival time difference is produced by the different routes for two split parts of the ultrashort light pulse to pass through, so that there is no need to mechanically vary the variable delay as in the conventional device of FIG. 1, and consequently it is possible to deal with a single optical signal, without requiring the light pulses to be generated stably for a number of times repeatedly.
However, this conventional device of FIG. 2 is also associated with the problem that it requires a relatively large light pulse optical power because of its use of the second harmonic generation which is the nonlinear optical phenomenon, Just as in thee conventional device of FIG. 1 described above.
Consequently, it is still difficult to utilize this conventional device of FIG. 2 for a highly sensitive reception of a single optical signal in the optical communication system.